1. Field of the Invention
The present invention relates to inorganic fiber and a method of producing the same. More particularly, it relates to inorganic fiber which has excellent heat resistance and, even if taken into human bodies, is easily discharged.
2. Description of the Related Art
Inorganic fibers, such as heat-resistant glass fiber and ceramic fiber, are widely used for heat insulation in various structures such as buildings because of their high heat resistance.
The inorganic fibers are liable to float in air in the preparation, storage, transportation or use and can enter human bodies through inhalation. Some of relatively large fibers that enter human bodies are trapped in the nasal cavity or the oral cavity and discharged out of the body together with snivel or saliva, and others are swallowed and go to digestive organs such as a stomach. However, fine fibers, not being caught by the mucous of the nasal or oral cavity, reach lungs and adhere to alveoli (a balloon-like organ performing exchange oxygen for carbon dioxide between air and blood).
In general, if foreign matter enters alveoli, alveolar macrophages surround the foreign matter and transport it to the places having cilia, such as the trachea and bronchi, so that the foreign matter can be driven out with phlegm, or the foreign matter is expelled from the surface of alveoli through lymph and lymphatic vessels. However, cases are sometimes met with in which alveolar macrophages are irritated or damaged by foreign matter. As a result, protease and a collagen fiber decomposing enzyme come out of the cells, and the alveolar cells suffer inflammation or become collagen in the presence of a large amount of such enzymes. Since cells suffering from inflammation have weakened resistance, DNA in the cell nuclei are susceptible to damage, and the cells frequently alternate destruction and regeneration, affording abnormal cells opportunities to develop. It may follow that denaturation of DNA cells or development of cancer cells are induced.
Hence, inorganic fibers for this kind of use have recently come to be required to have biodegradability, namely, biosolubility in physiological fluids such as pulmonary fluid, as well as heat resistance. Various inorganic fibers have been developed to date to meet these requirements. For example, JP-W-10-512232 (WO97/16386) discloses glass fiber having heat resistance to 1260xc2x0 C. and non-durability against physiological fluids which comprises silica (SiO2) and magnesia (MgO) as essential components and zirconia (ZrO2) as an optional component.
However, an inorganic fibers which reach alveoli usually have a fiber length of about 20 to 100 xcexcm, being so larger than general foreign matter such as viruses and bacteria that alveolar macrophages often fail to trap them. Besides, solubility of the conventional inorganic fibers is not higher than about several tens to several hundreds [ng/cm2xc2x7hr], and dissolution proceeds uniformly overall. Therefore, it takes a considerable time for the inorganic fibers to reduce in length to such a degree that alveolar macrophages may surround definitely without incurring irritation or damage, and there is a great possibility that the alveolar macrophages suffer damages meanwhile.
An object of the present invention is to provide an inorganic fiber having excellent biodegradability as well as heat resistance equal or superior to that of conventional inorganic fibers. Another object of the invention is to provide a method of producing such an inorganic fiber.
The present invention accomplishes the above object by providing an inorganic fiber which comprises SiO2, MgO and TiO2, as essential components, and has an amorphous portion comprising these components in its structure.
The present invention also accomplishes the above object by providing a method of producing inorganic fiber which comprises melting a raw material comprising SiO2, MgO and TiO2, as essential components, by heating at 1700 to 2000xc2x0 C. and fiberise the melt by quenching.